A new study released by the University of Michigan Addiction Center reports some troubling news about the state’s medical cannabis users. One in five participants said they had driven a car “very high” within the last half-year.

“There is a low perceived risk about driving after using marijuana, but we want people to know that they should ideally wait several hours to operate a vehicle after using cannabis, regardless of whether it is for medical use or not,” said Erin E. Bonar, the report’s lead author, assistant professor of psychiatry, and clinical psychologist. “The safest strategy is to not drive at all on the day you used marijuana.”

But that is decidedly not the strategy being practiced by the study’s participants. Over 50 percent of respondents — all of whom take cannabis for chronic pain issues — said they have driven high within two hours of consuming cannabis.

The study is particularly significant given the sheer number of cannabis users in Michigan. California is the only state with more medical marijuana patients in the United States. Overall, Michigan is home to some 270,000 medical cannabis users. Since the study was conducted, voters in the state approved widespread recreational marijuana usage, making it legal for anyone over the age of 21 to use cannabis and grow up to 12 plants for personal consumption.

Bonar’s study aggregated responses from 790 adults who applied for certification or recertification as a medical marijuana patient in 2014 and 2015. The report was funded by the National Institute of Drug Abuse and published in the Drug & Alcohol Dependence journal.

In Michigan it is illegal to drive under the influence of marijuana, regardless of the conditions under which it was consumed. The study’s results underline what may be a miscommunication about medical patients’ attitudes regarding the legal ramifications of their prescriptions. Participants may be unaware of the (very real) physical limitations that occur after consumption.

Bonar attributes this discordance between legality, safety, and reality with confusion over the way marijuana affects us and the ways in which it can be measured. “With alcohol, you can do some quick math based on the amount you drank, and take an educated guess at your blood alcohol level,” she told the University of Michigan Health Lab. “For marijuana, an estimate like this would be complicated. It’s hard to quantify because there is a lot of variation in marijuana dosing, THC potency, and route of administration. We also don’t have specific guidelines yet about when exactly it would be safe to operate a vehicle.”

Michigan is not the only state struggling with keeping cannabis users and those around them safe on the road. In Massachusetts, a special commission recommended lawmakers to implement “open container” laws that would ban people from carrying containers of cannabis in their vehicle, among other measures to make roads safe in the era of marijuana.